The invention relates to the production of coatings on a substrate.
More particularly, the invention relates to production of coatings having an outermost ceramic layer and functional and compositional gradients between the outermost ceramic layer and the substrate. The inventive functionally graded coatings provide protection from heat, oxidation, corrosion, erosion and wear of parts such as, for example, gas turbines or internal combustion engines.
Over the last 10 to 15 years improvement of protective coatings has been aimed at creation of coatings having specific discrete gradients or layers of coating composition and coating structure from the substrate to the coating upper layer. Characteristic examples of these coatings can be the thermalbarrier coatings deposited on metal substrates wherein the coating has a discrete, layered variation in composition across the thickness of the coating from the substrate to an outer ceramic layer. Such layered graded coatings are produced in several stages, using various materials (metals, alloys, ceramics) and technological processes for each layer.
U.S. Pat. No. 4,401,697 of Aug. 30, 1983 (T. E. Strangman) describes a three-layered thermal-barrier coating which consists of a bond coat of a oxidation resistant and corrosion resistant alloy of MCrAlY type material of 25-250 xcexcm thickness, an outer ceramic layer consisting of stabilized ZrO2 with a columnar structure and an interlayer of Al2O3 of 0.25-2.5 xcexcm thickness. The MCrAlY alloy comprises, for instance, 18 wt. % Cr, 23 wt. % Co, 12.5 wt. % Al, 0.3 wt. % Y, the remainder being Ni. The MCrAlY alloy bond coat is deposited onto the metal substrate surface by electron beam evaporation of an ingot of MCrAlY alloy. Then the coated surface is subjected to mechanical treatment (for instance, shot peening); the part is annealed and again placed into the vacuum chamber. The outer ceramic coating is produced by electron beam evaporation of a ceramic ingot of stabilized ZrO2 oxide and deposition of the vapor onto the bond coat. A thin interlayer of Al2O3 is subsequently produced during annealing of the coated substrate in an oxygen-containing atmosphere. This interlayer provides good adhesion on Al2O3/ZrO2 interface and slows down the oxidation of MCrAlY surface under high temperature service of the coating.
U.S. Pat. No. 4,676,994 of Jun. 30, 1987 (R. E. Demaray) recommends a four-layer coating which consists of a MCrAlY type alloy bond coat, a thin intermediate layer of Al2O3 and an outer two-layer coating of stabilized ZrO2. The MCrAlY bond coat of approximately 120 xcexcm thickness may be produced by electron beam evaporation of a MCrAlY ingot. After appropriate mechanical treatment of the surface (grit blasting), the part is placed into a vacuum furnace at a pressure of approximately 2xc3x9710xe2x88x924 mm Hg, heated to 980xc2x0 C. and soaked for about 10 minutes. This results in formation on the MCrAlY surface of an Al2O3 containing layer 1.0-2.0 xcexcm thick. Then a ceramic ingot of ZrO2 is evaporated by the electron beam under a vacuum of 10xe2x88x924 mm Hg and a dense (≈94% ) layer of ZrO2 approximately 50 xcexcm thick, is deposited. Subsequently, oxygen is bled into the chamber, and at the pressure of 5xc3x9710xe2x88x921 to 1xc3x9710xe2x88x923 mm Hg deposition of a less dense upper layer of ZrO2 with a columnar structure approximately 100 xcexcm thick, is completed. This layer has a lower heat conductivity and satisfactory mechanical relaxation ability.
U.S. Pat. No. 4,880,614 of Nov. 14, 1989 (T. E. Strangman et al) describes a thermal-barrier coating that consists of five layers. The first layer deposited on the substrate comprises diffusion aluminides produced by known processes. The second layer is of an alloy of the MCrAlY type deposited by electron beam evaporation or other methods. The third layer is a thin layer of super pure alpha Al2O3 produced by chemical vapor deposition (CVD). The fourth layer is a stabilized ZrO2 ceramic layer with a columnar structure, or other ceramics, deposited by electron beam evaporation. The fifth layer, a hard, dense, glazed outer ceramic layer, is produced by laser melting of the edges of the columnar crystallites. The fifth layer is intended to increase erosion resistance. The layer of diffusion aluminides and MCrAlY layer are intended to increase the oxidation and corrosion resistance of thermal-barrier coatings and thus to extend the service life of the coated part.
In European Patent EP 0814178 (D. S. Rickerby) a thermal-barrier coating consisting of seven layers is described. The first layer is a surface of nickel or cobalt base superalloy enriched in a metal of the platinum group, predominantly platinum. It is produced by deposition of a platinum layer 5-8 xcexcm thick by electroplating and subsequent diffusion annealing in the temperature range of 800 to 1200xc2x0 C. The second layer, a bond coat is made of an alloy that contains aluminum in the amount of 5-40 wt. %, for instance, MCrAlY type alloys or nickel aluminides or cobalt aluminides. The bond coat is deposited by a vacuum plasma process. The third and fourth layers are an enriched with platinum (or another metal of the platinum group) bond coat and a layer of platinum aluminide (or another aluminide), respectively. These layers are produced by electroplating of platinum or another metal of the platinum group on the bond coat surface and subsequent annealing of the plated substrate in the temperature range of 1000-1200xc2x0 C. The fifth, sixth and seventh layers are thin layers of gamma phase alumina which is enriched in platinum, a thin layer of pure alumina and an upper ceramic layer of yttrium-stabilized zirconia with a columnar structure, respectively. They are produced using repeated thermal cycles of electron beam evaporation and deposition of ceramic material followed by oxygen bleeding into the vacuum chamber.
U.S. Pat. No. 5,891,267 of Apr. 6, 1999 (J. C. Schaeffer et al) proposes a four-layer coating. The first layer is produced by carbidization of the substrate surface using superalloys which contain carbide forming elements, namely Mo, W, Re, Ta, Ti, Cr, Hf, Zr. Carbidization is performed using conventional furnaces in a mixed atmosphere of hydrogen and methane at lowered pressure and temperature of 900-1200xc2x0 C. for one to four hours. The first layer, saturated with carbon, has a thickness of up to 100 xcexcm and contains 25-75 vol. percent carbides. It is followed by a second layer, namely an aluminum-rich bond coat of diffusion aluminum or MCrAlY type alloy produced by known methods. The third, thin layer of Al2O3 and the fourth ceramic layer of ZrO2-(6-8) wt. % Y2O3 with a columnar structure are also produced by known methods, typically physical vapor deposition.
A characteristic feature of the above examples, as well as many other patents that have not been cited, is the multi-stage processing required for production of the layered gradient protective coatings. Typically there is a need to use 2, 3 or more technologically different processes involving different equipment and handling therebetween. Additionally, intermediate treatments of the layer surfaces between stages are required. As a result, known processes for forming layered functionally graded coatings require considerable power consumption, time and expense. Additionally, it is difficult to precisely repeat all of the process parameters for each of the required complex steps in known processes for forming layered functionally graded coatings. Variation of process parameters in any of the stages during the involved multi-step processing results in a low probability of complete repeatability of coating composition and structure; i.e., of coating quality, from part to part. Further, the known coating technologies between the metal and ceramic layers cannot be regarded as optimal in terms of producing flat interfaces. In terms of performance of the ceramic layers, smoothly varying transitions from metal to ceramics are preferable.
Further improvement of the methods of production of multi-layer gradient protective coatings is needed to reduce the number of process stages while also providing simultaneous formation of a continuously varying transition between layers, especially on metal/ceramic interfaces.
The solution closest to the present invention is that described in the U.S. Pat. No. 5,834,070 of Nov. 10, 1998 (B. A. Movchan et al). The ""070 patent proposes use of a composite ingot for producing by evaporation a functionally graded coating with an outer ceramic layer on a metal substrate. The composite ingot consists of a ceramic ingot of ZrO2(Y2O3) with a metal ceramic tablet located on the composite ingot upper face. The tablet consists of a mixture of metals and oxides having different vapor pressures at the tablet evaporation temperature. According to the ""070 reference, electron beam evaporation of the above ceramic ingot of ZrO2(Y2O3) and metal-ceramic tablet, followed by deposition of the vapor on a substrate, produces a coating with a gradient transition zone between the bond coat surface and outer ceramic layer of ZrO2(Y2O3).
The tablet mixture can be, for instance Alxe2x80x94Al2O3xe2x80x94ZrO2 or Alxe2x80x94Al2O3xe2x80x94Ptxe2x80x94ZrO2. The vapor pressure of the tablet components at the evaporation temperature is maximal for aluminum and minimal for zirconium oxide; i.e., it decreases in the following sequence:
Alxe2x86x92Al2O3xe2x86x92ZrO2.
Therefore as the composite ingot is heated, aluminum is the first material to evaporate, later accompanied by evaporation of aluminum oxide. At the final stage of heating the zirconium oxide of the tablet evaporates with an uninterrupted transition to evaporation of the zirconium oxide of the ingot. As a result, a gradient transition zone (bond coat) 3-5 xcexcm thick forms between the substrate surface and the outer ceramic coating of ZrO2(Y2O3) during vapor condensation onto the substrate. The gradient transition zone consists of individual microlayers, for instance NiAl, Al2O3, or Al2O3xe2x80x94ZrO2. It should be noted that condensation of the tablet vapor flow proceeds, as a rule, onto the substrate surface that has been preheated to temperatures above the aluminum melting point (660xc2x0 C.). Therefore, the first portions of aluminum to condense on the substrate surface are in the form of a very thin layer of liquid which interacts with the material of the substrate surface or previously applied MCrAlY bond coat, and provides a strong bond between the substrate and the gradient transition zone. As was noted above, the xe2x80x9cmetal ceramic tablet/ceramic ingotxe2x80x9d composition allows limited formation of a gradient transition zone (bond coat) between the metal substrate and the outer ceramic coating. The composition and structure of this gradient transition zone is dependent on the tablet metal-ceramic mixture that is fractionally evaporated by electron beam heating. The requirement for fractional evaporation of tablet metal-ceramic mixtures imposes considerable limitations on the resulting coating composition and structure, as well as on the thickness of the gradient transition zone. Moreover, the above approach does not permit creation of many of the desired gradients of composition or structure, either of the metal bond coat or of the upper ceramic layer.
An object of the invention is to provide a composite ingot for use in formation of a functionally graded coating on a substrate.
Another object of the invention is to provide a composite ingot for use with a single stage coating process to form a functionally graded coating on a substrate.
Yet another object of the invention is to provide a composite ingot that can be continuously and sequentially evaporated and the vapors condensed on a substrate to form a functionally graded coating on a substrate.
A further object of the invention is to provide a single stage coating process for formation of a multilayer coating on a substrate.
A still further object of the invention is to provide a single stage coating process of improved precision and repeatability for formation of a coating having a desired gradient composition and gradient structure on a substrate.
Other objects and advantages of the invention will become apparent from the drawings and the specification.
One aspect of the invention comprises a composite ingot that can be evaporated and the vapors condensed on a substrate to form a functionally gradient coating with an outer ceramic layer on the metal substrate. The composite ingot comprises a ceramic body that preferably has a predominately cylindrical shape. At least one, and more preferably multiple, inserts are partially or fully disposed within the upper, middle and/or lower parts of the body. The inserts are comprised of metallic materials, nonmetallic materials or mixtures thereof. The selection and appropriate arrangement of the ceramic body and the inserts provides for formation of a gradient multilayer coating having a desired composition and structure when the composite ingot is evaporated and the vapors deposited on a substrate. It is preferable for the above inserts to have the shape of tablets or bars with a cylindrical, conical or more complex-shaped surface.
Another aspect of the invention is a single stage coating formation process wherein a composite ingot is continuously evaporated and the vapors condensed on a substrate to form a functionally gradient coating with an outer ceramic layer on the metal substrate. Preferably the composite ingot is heated from a first surface to a second surface via a concentrated energy source, for example an electron beam. The evaporation of the ingot is preferably substantially continuous from the first surface to the second surface. As the temperature of the heated surface increases materials evaporate from the heated surface and the vapors condense and deposit onto an adjacent substrate surface. As the composite ingot evaporates, different portions of the composite ingot are exposed to the electron beam and in turn begin to evaporate. The different composite ingot portions have different compositions depending on the material of the ingot body and location and material of the inserts within the body. The differing ingot compositions when evaporated provide varying vapor compositions. The varying vapor compositions deposit on the substrate surface to form a gradient multilayer coating having a desired composition and structure.
Preferably, the inserts located partially or fully within the upper part of the composite ingot (the first part of the composite ingot to be heated) are made of materials which have a melting temperature lower than, and a vapor pressure higher than, the melting temperature and vapor pressure of the ceramic body of the ingot. In this way these inserts are the first to evaporate on heating of the composite ingot, and their vapors preferentially condense on the substrate to form a transition bond coat or layer of the desired composition and structure. One should emphasize the positive influence of the low melting metals and alloys present in the inserts, whose melting temperature is preferably lower than the preheated temperature of the substrate prior to coating deposition. In this situation a thin molten film is created on the substrate surface at the initial moment of insert vapor condensation. The molten film dissolves the substrate surface microroughness, interacts with the substrate material and promotes the formation of a dense structure of the substrate bond coat contact zone.
Preferably, the materials of the inserts located in the upper part of the ceramic base are selected depending on their coating purpose. The inserts may comprise metals, alloys, intermetallics, silicides, metal ceramics, or organic substances so that in heating of the composite ingot, the material of the inserts is the first to evaporate and the first to condense on the substrate, forming a transition bond coat layer (or layers) of the desired structure and composition on the substrate. It is this ability to select materials which functions to provide the desired structure and composition of the transition bond coats on the substrate.
It is preferred for the inserts located in the middle and lower part of the ceramic body (the middle and last parts of the composite ingot to be heated respectively) to be made predominantly from a nonmetallic material. In this manner the material of the inserts evaporates and condenses simultaneously with the ceramic material of the body, providing the desired composition and structure for the middle and outer ceramic layers of the gradient coating.
One embodiment is especially suited to produce a thermal-barrier functionally graded coating with an outer ceramic layer on a metal substrate using a single stage coating process. In this embodiment the ceramic body of the composite ingot is comprised of a partially or completely stabilized ZrO2 and contains inserts which are located in the upper, middle, and lower parts of the composite ingot. The ceramic body is comprised of ZrO2, which has a low thermal conductivity and provides the thermal-barrier properties of the resulting gradient coating. The inserts are comprised of metallic or nonmetallic materials having shapes and dimensions suitable for formation of the desired coating. The inserts in the composite ingot function to ensure formation of a desired gradient multilayer coating of the specified composition and structure during a continuous single-stage evaporation of the composite ingot and condensation of the evaporated vapors onto the substrate. It is preferred that in this embodiment the inserts have the shape of tablets or bars with a cylindrical, conical or more complex-shaped surface.
It is preferable that when the ingot body comprises ZrO2, the inserts located in the upper part of the composite ingot are made of a material comprising Al, Si, Fe, Ni, Co, Cr, Mn, Y, Pt, Zr, Hf, AlCr alloys, M1Cr and M1CrAlY type alloys (where M1xe2x95x90Fe, Ni, Co), nickel aluminides, cobalt aluminides, platinum aluminides, and their alloys, chromium silicides, carbon-containing organic compounds, Al2O3, Cr2O3, La2O3, CeO2, B2O3, MgO, metal-ceramic mixtures of M2xe2x80x94Yxe2x80x94ZrO2, M2xe2x80x94Yxe2x80x94Ptxe2x80x94ZrO2, M2xe2x80x94Yxe2x80x94Al2O3xe2x80x94ZrO2 type (where M2xe2x95x90Al, Cr), Al2O3, Cr2O3, Y2O3, ZrO2 and mixtures of any of the above. These materials and combinations function to promote the formation of a desired structure of the substrate bond coat contact zone and provide high adhesion to the overlying ZrO2 thermal barrier coating.
It is preferable in this embodiment that the inserts located in the middle and lower part of the composite ingot to be made of a material comprising Al2O3, Y2O3, La2O3, B2O3, CeO2, HfO2, MgO, CaO, SiO2 and mixtures thereof. These materials and combinations function to allow a smooth transition zone to be formed between the metal bond coat layer (or layers) and the overlying ceramic layer (or layers) of the ZrO2 based gradient coating.
A second embodiment of the invention is especially suited to produce high temperature and erosion resistant functionally graded coatings with an outer ceramic layer on a metal substrate using a single stage process of coating deposition. In this embodiment the ceramic body of the ingot is made from Al2O3 and contains inserts which are located in the upper, middle and lower parts of the body. The inserts are made of metallic or non-metallic materials and have the shapes and dimensions suitable for formation of the desired coatings as described below. This embodiment provides for production of not only high temperature and erosion-resistant, but also hard and wear resistant Al2O3 based coatings. It is preferred that in this embodiment the above inserts have the form of tablets or bars with a cylindrical, conical or more complex-shaped surface.
It is preferable that when the ingot body is made of Al2O3, the inserts located in the upper part of the body are made from a material comprising Sn, Al, Cu, Fe, Ni, Co, Cr, Y, M3Cr and M3CrAlY alloys (where M3xe2x95x90Sn, Cu, Fe, Ni, Co), iron, nickel and cobalt intermetallics, chromium silicides, carbon-containing organic compounds, M4xe2x80x94Al2O3, M4xe2x80x94Nixe2x80x94Al2O3 (where M4xe2x95x90Sn, Al, Cr, Y, Fe, Cu), Snxe2x80x94Crxe2x80x94Al2O3 metal ceramic mixtures and mixtures of any of the above. These materials and combinations function to provide the desired structure of the substrate bond coat contact zone and provide high adhesion of the high temperature and erosion resistant functionally graded Al2O3 based coatings.
It is preferable for the inserts located in the middle and lower part of the composite ingot to be made of a material comprising Cr2O3, MgO, SiO2, ZrO2, Y2O3, B2O3 and mixtures thereof. These materials and combinations function to allow a smooth transition zone to be formed between the metal bond coat layer (or layers) and the overlying ceramic layers of the Al2O3 based gradient coating.
Another aspect of the invention is a composite ingot and process to produce a desired hard and wear-resistant functionally graded coatings with an outer ceramic layer on a metal substrate by a single stage coating process. A third embodiment of the invention is especially suited to produce a functionally graded coating with an outer ceramic layer on a metal substrate using a single stage deposition process. In this embodiment the ceramic body of the ingot is made from TiC and contains inserts which are located in the upper, middle and lower parts of the body. The inserts are made of metallic or non-metallic materials having shapes and dimensions suitable for formation of the desired coating as described below. The inserts in the composite ingot function to ensure formation of a desired gradient multilayer coating of the specified composition and structure during a continuous single-stage evaporation of the composite ingot and condensation of the evaporated vapors onto the substrate. It is preferred that in this embodiment the inserts have the form of tablets or bars with a cylindrical, conical or more complex shaped surface body. It is preferable that when the ceramic body of the ingot is made of TiC, the inserts located in the upper part of the composite body are made from a material comprising Sn, Al, Cu, Fe, Ni, Co, Cr, M5Cr, M5CrAl alloys (where M5xe2x95x90Sn, Cu, Fe, Ni, Co), NiCo, carbon containing organic compounds, Coxe2x80x94TiC; Nixe2x80x94TiC, Crxe2x80x94Coxe2x80x94TiC, Crxe2x80x94Nixe2x80x94TiC, Snxe2x80x94Crxe2x80x94Ni(Co)xe2x80x94TiC, Snxe2x80x94Crxe2x80x94Tixe2x80x94TiC and mixtures of any of the above. These materials and combinations function to promote the formation of a desired structure of the substrate bond coat contact zone and provide high adhesion of the overlying high-temperature and erosion resistant functionally graded TiC base coatings.
It is preferable in this embodiment for the inserts located in the middle and lower part of the composite ingot to be made of a material comprising ZrC, HfC, Cr3C2, TiB2 and mixtures thereof. These materials and combinations function to allow a smooth transition zone to be formed between the metal bond coat layer (or layers) and the overlying ceramic layer (or layers) of the gradient TiC base coating.
A fourth embodiment of the invention is especially suited for production of hard and wear-resistant functionally graded coatings with an outer ceramic layer on a metal substrate using a single stage process. In this embodiment the ceramic body of the ingot is made of TiB2 and contains inserts which are located in the upper, middle and lower parts of the composite ingot. The inserts in the composite ingot function to ensure formation of a desired gradient multilayer coating of the specified composition and structure during a continuous single-stage evaporation of the composite ingot and condensation of the evaporated vapors onto the substrate. The inserts are made of metallic or non-metallic materials having shapes and dimensions suitable for formation of the desired coating as described below. It is preferred that in this embodiment the inserts have the form of tablets or bars with a cylindrical, conical or more complex-shaped surface.
It is preferable that when the ingot body comprises TiB2, the inserts located in the upper part of the composite ingot are made from a material comprising Sn, Al, Cu, Fe, Ni, Co, Cr, M6Cr type alloys (where M6xe2x95x90Sn, Cu, Fe, Ni, Co), cobalt silicides, carbon-containing organic compounds, Crxe2x80x94TiB2 Snxe2x80x94Tixe2x80x94TiB2, Snxe2x80x94Crxe2x80x94TiB2, Snxe2x80x94Tixe2x80x94TiB2 and mixtures of any of the above. These materials and combinations function to provide the desired structure of the substrate bond coat contact zone and provide high adhesion of the high temperature and erosion-resistant functionally graded TiB2 based coatings.
It is preferable in this embodiment for the inserts located in the middle and lower part of the composite ingot to be made of a material comprising ZrB2, TiC, ZrC, HfC and mixtures thereof. These materials and combinations allow a smooth transition zone to be formed between the metal bond coat layer (or layers) and the overlying ceramic layer (or layers) of this gradient TiB2 based coating.
In any embodiment, formation of a complex multi-phase ceramic coating outer layer is achieved by placing in the ingot lower portion a tablet comprised of non-metallic materials with a broad range of melting temperatures and vapor pressures. The tablet is the last to evaporate and completes the formation of the gradient coating. In particular, the tablet composition and process condensation conditions determine the degree of coating surface roughness.
The composite ingot is produced by traditional metallurgical methods, primarily powder metallurgy methods. These production methods allow precise control over the shapes and composition of the ceramic body and the shapes, compositions and locations of the inserts within the body. The preferred single stage coating deposition process wherein the composite ingot is sequentially evaporated from a first side to an opposing side produces a vapor having varying chemical compositions over the time period of the evaporation. Deposition of the vapor onto the substrate is continuous with evaporation so that a coating having a desired gradient composition and gradient structure from the substrate to the coating outer surface can be produced. The precision and repeatability of both the composite ingot and the preferred single stage evaporation-deposition process provide a high level of repeatability of the composition, structure and properties of the resulting functionally graded coatings.